Continuous pulp digester



g 5, 1952 R. G. LEWlS I 2,606,114

CONTINUOUS PULP DIGESTERI Filed NOV. 1., 1946 5 Sheets-Sheet l P0661 4 6 ew/"s" 1952 R. G. LEWIS 2,606,114

CONTINUOUS PULP DIGESTER Filed Nov. 1, 1946 3 Sheets-Sheet 2 7% zzw 1952 R. G. LEWIS I 2,606,114

CONTINUOUS PULP DIGESTER h. 7 [ml W WIIIIIIIIIIIIIIIIII Patented Aug. 5, 1952 CONTINUOUS PULP DIGES-TER Robert G. Lewis, Chippewa Falls, Wis., assignor of one-half to Sidney D. Wells, Combined Locks,

Wis.

Application November 1, 1946, Serial No. 707,189

7 Claims.

This invention relates to a continuous pulp digester for fibrous material in the making of paper and the like.

The invention has been applied to the construction of a digester particularly adapted to be employed in the continuous paper pulp making process of the type described in U. S. Letters Patent No. 2,029,973 granted to S. D. Wells on February 4, 1936. In this process the partially cooked and prepared material to be digested is composed of vegetable fibers such as cornstalks, cereal straws, fiax straw, bamboo, bagasse and the like.

Heretofore, practically all commercial forms of digesters have been of the batch type and have not fitted well into a continuous process system. Attempts to employ continuous digesters have generally resulted in either too expensive capital investment or too expensive and wasteful process treatment to be commercial.

One of the principal objects of the present invention is to provide a digester which is economical and which can be commercially used without excessive cost either of the installation or of the process.

Another object of the invention is to provide a continuous digester capable of adjustment of operating conditions to effect the required digestion of any of the materials referred to, and if desired, of wood.

Another object is to provide a continuous digester in which the temperature, pressure and time of throughput of the stock can be readily controlled to provide the optimum results desired Another object is to provide a continuous digester in which the heat is conserved by a heat exchange between the outgoing digested pulp and the incoming white liquor.

Another object is to provide a continuous digester in which the danger of steam flashing is eliminated.

Other objects and advantages will appear in the following description of an embodiment of the invention illustrated in the accompanying drawings.

In the drawings:

Figure l is a schematic showing of the digester, the heat exchanger, and the connections thereto;

Fig. 2 is an enlarged detail broken section of one of the digesters, illustrating its construction; and

Fig. 3 is a similar section of the heat exchanger.

The apparatus comprises, in general, two digesters i and 2 and a heat exchanger 3 arranged horizontally with digester 2 above digester I. If desired, the digesters may be disposed end to end in the same shell, athough it is preferred to keep them separate for simplification of construction and installation and more flexible control of operation. Additional digesters may be provided where longer cooking and higher temperatures are desired, and a single heat exchanger may be of sufiicient size to operate with two sets of digesters.

The system involves the continuous movement of the pulp stock material from end to end of digester i, then from end to end of digester 2, and finally, from end to end of heat exchanger 3. Steam is passed through the digesters in a direction counter-current to the flow of stock to effect cooking of the same so that the stock leaving digester 2 is the hottest. White liquor is passed through heat exchanger 3 in a direction countercurrent to the fiow of stock so that the latter is lowered in temperature when it leaves the heat exchanger to below steam temperature at atmospheric pressure to thereby prevent steam flash in the stock when the pressure is released.

Each digester l and 2, as showrncomprises a stationary jacketed vessel of the order of thirty or more feet in length and about five feet in diameter. The jacket of each digester is composed of the inner digester shell 4 and an outer shell 5 radially spaced therefrom and between which is a spiral partition 6 providing a spiral steam passage from end to end of the digester.

An internal rotary steam drum 8 of abo u't thirty inches in diameter extends axially, longitudinally of each digester and has a plurality of stirring blades 9 extending radially outward therefrom with the pitch of the blades directed to assist in the movement of the stock through the digester as the drum rotates.

The drum 8 for each digester is mounted at its closed inner end in an internal bearing ID in rear head I! of the digester. The opposite end of drum 8 has a shoulder [2 bearing against the end head 13 of the digester and a reduced end I4 of smaller diameter is supported by a stuffing box and bearing !5 and extends through head 13 to the outside of the digester.

The drums 8 are rotated at a suitable speed, such as twenty revolutions per minute, by means of the cog wheels IS on the end of the corresponding trunnion-like extensions [4. The cog wheels It are driven by chains or other means from a motor, not shown.

The trunnion end Id of the drum 8 for digester l contains a separator for condensate which may be constructed in any suitable manner. The separator shown comprises a siphon pipe ll, having its inlet end disposed on the inside of the drum 8 near the bottom where condensate gathers, and which extends outwardly axially of trunnion l4 through the wall of a steam pipe connection It and downwardly on the outside to create the siphon eiiect. A packed joint 19 seals the pipe I! in the wall of connection 18.

The connection 3 is clamped to a flanged extension fitting 20 on trunnion l4 and a rotary steam seal 2| is provided between the connection and the trunnion. The steam supply pipe 22 supplies steam to drum 8 through connection I8 and the axial bore of trunnion Id and its extension 20 which is substantially larger in diameter than pipe I! to provide a passage therethrough for the steam.

Condensate is withdrawn from the jacket of digester I by a drain pipe 23 secured to the bottom of the outer shell 5 near the head I3 and communicating with the end of passage I at the bottom. Each convolution of spiral partition 6 has a notch 24 therein at the bottom for the free flow of condensate along the bottom of shell 5 to the drain pipe 23.

v Steam is admitted to the spiral passage I in the jacket of drum I by means of the pipe 25 connecting therewith directly through shell 5 at the rear end of the digester near the head II.

The pulp is pumped into the chamber of digester I between the shell 4 and drum 8 by a pump 26 through pipe 2? connecting with the chamber through head I 3 of the digester.

The pulp is discharged from the chamber of digester I through a pipe 8 connecting therewith through the rear end head II and conducting the same to a booster pump 29.

The digester 2 has a construction similar to that for digester I except for differences in the flow of material.

The pulp is supplied to digester 2 from booster pump 29 to the rear end of the digester chamber by pipe 38 connecting through the head II.

The digested pulp is discharged from the chamber of digester 2 through the pipe 3I connecting therewith through head I3 and leading to the heat exchanger 3.

The blades 9 on drum 3 of digester 2 may be pitched in the opposite direction from those for digester I, or the drum 8 of digester 2 may be rotated in the opposite direction to that for drum 8 of digester I', in order to obtain the desired direction of fiow of pulp from the rear end to the front end of digester 2 as distinguished from the flow of pulp from the front end to the rear end of digester I.

Live steam is supplied to the spiral passage I in the jacket of digester 2 through a branch pipe 32 connecting through shell 5 near head 53 and leading from a header steam supply pipe 33. A similar branch pipe 34 leading from supply pipe 33 supplies live steam to drum 8 of digester 2 through the connection I8. Valves 35 are placed in each of the branch pipes 32 and 34 to regulate the proportion and amount of steam entering the jacket and. the drum.

Steam and condensate are discharged from the jacket of digester 2 through the pipe 25 connecting with the bottom of the jacket near head H and which conducts the steam directly'to the jacket of digester I as previously described,

Steam and condensate are discharged from drum 8 of digester 2 through the corresponding condensate collector pipe I I. The steam discharge pipe I! for digester 2 constitutes the steam supply pipe for digester I and connects directly with the pipe 22 of digester I.

The heat exchanger 3 is utilized to conserve the heat in the digested pulp by transferring the heat to the white liquor, which is usually a caustic soda solution, prior to mixture of the latter with the dry or wet raw pulp stock which is to be pumped into digester I.

The heat exchanger 3 may have any suitable construction to accomplish the purpose stated,

that shown embodying a jacketed vessel of about the size of the digester I and having an inner shell 36 and an outer shell 37 between which there is a spiral partition 38 to provide a spiral passage 39 from end to end of the vessel for white liquor.

A rotary shaft 40 extends axially through the heat exchanger 3 and is supported in suitable bearings 4| in the end heads 42 and 43 of the heat exchanger. A suitable stufiing box 44 seals the shaft 43 relative to the heads 42 and 43. The shaft 49 has a reduced end extending through head 43 and which carries a sprocket 45 for driving the shaft.

The shaft 40 carries a plurality of interspaced spiral pipe coils 46 extending from end to end of the heat exchanger and which rotate with the shaft, the pitch of the spirals and their direction relative to the direction of rotation serving to assist in the advance of the hot pulp longitudinally through the heat exchanger from the front to the rear.

White liquor is supplied to the heat exchanger from the tank 47 through header pipe 48, and branch pipe 49 connecting through shell 31 ad- -jacent head 42 to spiral passage 39, and branch pipe 50 connecting axially through head 42 to a passage 5I on the inside of shaft 40.

The rear ends of coils 46 are turned radially inward and welded to shaft 40 at corresponding locations around the circumference. Radial passages 52 extend from longitudinal passage 5| in shaft 40 to connect with the corresponding coils 46 to supply white liquor thereto.

The front ends of coils 46 are similarly turned radially inward and welded to shaft 40 at corresponding locations around the circumference. Radial passages 53 in shaft 48 lead from the corresponding coils and conduct the white liquor therefrom to a longitudinal discharge passage 54 in the front end of the shaft.

The outer end of shaft 4t carries a stationary header chamber 55 into which passage 54 discharges the white liquor from coils 46. A suitable rotary seal 56 is provided between the rotary shaft 48 and the walls of chamber 55.

A pipe 5'! leads from the front end of spiral passage 39 near head 43 and discharges into header chamber 55.

The white liquor which is heated in exchanger 3 and discharged into header 55 is conducted through pipe 58 to the mixing tank 59 where it is mixed with incoming stock ready for pumping into digester I. A pipe 60 supplies the pulp from tank 59 to pump 26.

Valves GI are disposed in the branch pipes 49 and 59, respectively, to regulate the flow of white liquor through the heat exchanger 3 and to proportion the same between the passage 39 and the coils 46 so that the liquor being discharged into header 55 from each is at approximately the same temperature.

The hot digested pulp enters heat exchanger 3 through pipe 3| connecting with the main chamber of the heat exchanger through head 43. The pulp leaves the exchanger through a discharge pipe 82 connecting through head 42 and which has a valve 53 for controlling the discharge flow of pulp.

One example of the operation of the system is as follows:

A suitable cornstalh pulp mixture, containing about six to seven per cent solids and the usual amount of caustic soda, was pumped into digester l by pump 25 at a rate to maintain a pressure of about twenty-five pounds per square inch in the digester. Upon transfer of the pulp to digester 2 the booster pump 29 is set to maintain a pressure of about seventy-five pounds per square inch in the pulp in digester 2.

The pulp entered digester I at about 210 F. and was transferred to digester 2 at about 260 F. The pulp discharged from digester 2 was at about 320 F. and it entered heat exchanger 3 at about that temperature.

The pulp pressure maintained in heat exchanger 3 was about seventy pounds per square inch.

The temperature of the pulp discharged from heat exchanger 3 through pipe 62 was about 200 F., and was below the steam flash temperature at atmospheric pressure so that the pressure on the pulp could be released without danger of steam flash and resulting injury to the pulp.

The live steam entering digester 2 was at about 125 pounds per square inch pressure. It left digester 2 and entered digester I at about 100 pounds per square inch and was discharged from digester I as condensate.

The white liquor entered heat exchanger 3 at about 100 F. and was discharged therefrom into mixing tank 59 at about 220 F. The valves GI and 63 were manipulated to provide the required throughput of white liquor and pulp for heat exchanger 3 to provide the desired reduction in temperature of the pulp to below the steam flash temperature.

The pulp after digestion contained from three to four per cent solids.

The time of cooking of the pulp may vary with different materials. The time for passage of the pulp through the two digesters I and 2 in the example given above was about two hours.

The temperatures, pressures and flow conditions referred to may be varied for different raw materials, and may vary depending upon each other. In general, the time required for any given stock will depend upon the temperature employed. 7

The temperature to which the stock should be heated will depend upon the raw materials employed and should be suflicient to effect the desired digestion of substantially all non-fibrous constituents of the pulp by the caustic soda within the time provided. Too low a temperature for a given stock will leave the pulp somewhat raw and undesirable. Too high a temperature may unduly weaken cellulose fibers of the pulp.

The temperature may be varied by controlling the steam input or by controlling the rate of throughput of the stock for a given length of digester. I

For some raw materials it may be possible to employ a single digester of sufficient length to complete the digestion. For others it may be advisable to employ two or more shorter digesters at different temperatures and pressures. Fresh steam may be supplied to each of the vessels in parallel or it may be supplied to them in series, as shown.

The stock pressure in each digester should be sufficient to prevent steam flash at the highest temperature of the stock therein.

Various embodiments of the invention may be employed within the scope of the accompanying claims.

I claim:

1. A continuous process pulp digester comprising at least two stationary digester vessels having conduit connections for filling of the same with liquid pulp stock and the continuous passage of pulp stock therethrough in series, heat exchange means disposed within the vessels to heat the pulp stock in said vessels and thereby effect an initial heating and partial cooking of the pulp stock as it passes through the first vessel and a higher heating and further cooking in the second vessel and in any successive vessel until the pulp stock reaches an optimum temperature for substantially complete digestion of the non-fibrous constituents thereof without injury to the cellulose fibers, means disposed in said vessel connections to maintain the flowing pulp stock under-a pressure in each vessel corresponding to the highest temperature therein to prevent steam flash in the pulp stock, and a heat exchanger vessel means disposed to receive and cool said pulp stock upon discharge from the last vessel and prior to the reduction in pressure thereof to a temperature below that for steam flash at the reduced pressure.

2. A continuous process pulp digester comprising at least two stationary digester vessels having conduit connections for filling of the same with liquid pulp stock and the continuous passage of pulp stock therethrough in series, heat exchange means disposed within the vessels to heat the pulp stock in said vessels and thereby effect an initial heating and partial cooking of the pu p stock as it passes through the first vessel and a higher heating and further cooking in the second vessel and in any successive vessel until the pulp stock reaches an optimum' temperature for substantially complete digestion of the nonfibrous constituents thereof without injury to the cellulose fibers, a rotary pump disposedto supply pulp stock to be digested to said first vessel in a substantially continuous flow therefor and at a pressure substantially above that corresponding to the maximum temperature for the stock in said vessel to prevent steam flash therein, a corresponding rotary pump disposed in the conduit entering each succeeding vessel to step up the pressure on the pulp stock therein corresponding to the maximum temperature for the stock in the respective vessel to prevent steam flash therein, said pumps cooperating in series to maintain the pump under pressure at all times in its passage through the several vessels, means disposed to govern the rate of discharge of stock from the last vessel, and a heat exchanger vessel means disposed to receive and cool said pulp stock upon discharge from the last vessel'and prior to the reduction in pressure thereof to a temperature below that for steam flash at the reduced pressure.

3. A continuous process pulp digester comprising at least two stationary digester vessels having conduit connections for filling of the same with liquid pulp stock and the continuousj passage of pulp stock therethrough in series, heat exchange means disposed within the vessels to heat the pulpstock in said vessels and thereby effect an initial heating and partial cooking of the pulp stock as it passes through the first vessel anda higher heating and further cooking in the second vessel and in any successive vessel until the pulp stock reaches an optimum temperature for substantially complete digestion of the non-fibrous constituents thereof without injury to the cellulose fibers, said heat exchange means including means to flow steam serially in heat exchange relation to the pulp stock inthe several vessels and in a direction counter-current to the directicn of movement of the pulp stock through said vessels, a rotary pump disposed to supply pulp stock to be digested to saidifirst vessel in a substantially continuous flow therefor and at a pressure substantially above that corresponding to the maximum temperature for the stock in said vessel to prevent steam flash therein, a corresponding rotary pump disposed in the conduit entering each succeeding vessel to step up the pressure on the pulp stock therein corresponding to the maximum temperature for the stock in the respective vessel to prevent steam flash therein, a heat exchanger connected to receive the hot digested pulp from said last vessel and to transfer its heat to at least a part of the in coming stock, and means disposed to govern and correlate the rate "of discharge of stock from the heat exchanger and the throughput for said incoming stock to effect cooling of the digested pulp stock to a temperature below that for steam flash at the reduced pressure resulting from discharge of the pulp stock from the heat exchanger.

4. A continuous process pulp digester comprising at least two stationary digester vessels having conduit connections for filling of the same with liquid pulp stock and the continuous passage of pulp stock therethrough in series, heat exchange means disposed within the vessels to heat the pulp stock in said vessels and thereby eiTect an initial heating and partial cooking of the pulp stockas it passes through the first vessel and a higher heating and further cooking in the second vessel and in any successive vessel until the pulp stock reaches an optimumtemperature for substantially complete digestion of the nonflbrous constituents thereof without injury to the cellulose fibers, said heat exchange means including means to flow steam serially in heat exchange relation to the pulp stock in the several vessels and in a direction counter-current to the direction of movement of the pulp stock through said vessels, a rotary pump disposed to supply pulp stock to be digested to said first vessel in a substantially continuous flow therefor and at a pressure substantially above that corresponding to the maximum temperature for the stock in said vessel to prevent steam flash therein, a corresponding rotary pump disposed in the conduit entering each succeeding vessel to step up the pressure on the pulp stock therein corresponding to the maximum temperature for the stock in the respective vessel to prevent steam flash therein, and a heat exchanger connected to receive the hot digested pulp from said last vessel and to transfer its heat to at least a part or the incoming stock.

5. A continuous process pulp digester comprising at least one stationary substantially horizontal cylindrical digester vessel having conduit connections for the continuous passage of pulp stock'therethrough from one end to the other, a steam jacket for said vessel to heat the same, a spiral partition in said jacket to force the steam to travel circumferentially of the vessel during longitudinal travel of the steam, a port through said partition at the bottom of each convolution for the drainage of condensate from one space through the partition to another space, a' drain port connected through the bottom of the jacketto one of the spaces therein for drainageof condensate from the several spaces intermediate the convolutions of said partition, a rotary steam drum disposed axially of said vessel with its walls in direct contact with the pulp stock to .supply additional heat thereto, means connectedto the drum through one end of said vessel and disposed to rotate said steam drum to more uniformly heat the stock by the drum, means disposed in said vessel connections to effect a continuous flow of pulp stock through the vessel'from one end to the other and to maintain a pressure on said stock during digestion in said vessel to prevent steam flash therein, and means disposed to cool said stock upon discharge irom said vessel and prior to the release of pressure thereon to a temperature below that for steam flash at atmospheric pressure.

6. In a continuous process pulp digester system, a stationary jacketed generally cylindrical heat exchange vessel adapted to receive the hot digested pulp under pressure and to cool the same for discharge at a temperature below that for steam flash at atmospheric pressure, means connected to the vessel to feed the pulp thereto at one end thereof, a valve controlled discharge connection for the pulp at the other end of said vessel, means connected to the jacket of said vessel to supply a cooling medium thereto, a rotary heat exchange member having generally spirally arranged tubes and disposed to turn on the longitudinal axis of the vessel within the pulp space therein, connections to flow a cooling medium through said rotary member, and means disposed to rotate said member in a direction to urge the flow of pulp toward the discharge end of the vessel by means of said spirally disposed tubes.

7. In a continuous process pulp digester system, a stationary jacketed generally cylindrical heat exchange vessel adapted to receive the pulp under pressure, means connected to the vessel to feed the pulp thereto at one end thereof, a dis charge connection for the pulp at the other end of said vessel, means connected to the jacket of said vessel to supply a heat exchange medium thereto, a rotary heat exchange member having generally spirally arranged tubes and disposed to turn on the longitudinal axis of the vessel Within the pulp space therein, connections to flow a heat exchange medium through said rotary member, and means disposed to rotate said member in a direction to urge the flow of pulp toward the discharge end of the vessel by means of said spirally disposed tubes.

ROBERT G. LEWIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 40,696 Lyman Nov. 24, 1863 110,873 Sinclair Jan. 10, 1871 448,581 Kaffenberger Mar. 1'7, 1891 704,259 Holmes July 8, 1902 1,080,602 Stokes Dec. 9, 1913 1,679,336 Dunbar July 31, 1928 1,734,824 Offenhauser Nov. 5, 1929 1,856,453 Baccus May 3, 1932 1,938,802 Braun Dec. 12, 1933 2,007,348 Scharmann July 9, 1935 2,008,635 Brubacher July 16, 1935 2,028,419 Sperry Jan. 21, 1936 2,200,034 Merrill May 7, 1940 2,269,898 Anderson Jan. 13, 1942 2,355,091 McDonald Aug. 8, 1944 

1. A CONTINUOUS PROCESS PULP DIGESTER COMPRISING AT LEAST TWO STATIONARY DIGESTER VESSELS HAVING CONDUIT CONNECTIONS FOR FILLING OF THE SAME WITH LIQUID PULP STOCK AND THE CONTINUOUS PASSAGE OF PULP STOCK THERETHROUGH IN SERIES, HEAT EXCHANGE MEANS DISPOSED WITHIN THE VESSELS TO HEAT THE PULP STOCK IN SAID VESSELS AND THEREBY EFFECT AN INITIAL HEATING AND PARTIAL COOKING IN THE SECOND VESSEL AS IT PASSES THROUGH THE FIRST VESSEL AND A HIGHER HEATING AND FURTHER COOKING IN THE SECOND VESSEL AND IN ANY SUCCESSIVE VESSEL UNTIL THE PULP STOCK REACHES AN OPTIMUM TEMPERATURE FOR SUBSTANTIALLY COMPLETE DIGESTION OF THE NON-FIBROUS CONSTITUENTS THEREOF WITHOUT INJURY TO THE CELLULOSE FIBERS, MEANS DISPOSED IN SAID VESSEL CONNECTIONS TO MAINTAIN THE FLOWING PULP STOCK UNDER A PRESSURE IN EACH VESSEL CORRESPONDING TO THE HIGHEST TEMPERATURE THEREIN TO PREVENT STEAM FLASH IN THE PULP STOCK, AND A HEAT EXCHANGER VESSEL MEANS DISPOSED TO RECEIVE AND COOL SAID PULP STOCK UPON DISCHARGE FROM THE LAST VESSEL AND PRIOR TO THE REDUCTION IN PRESSURE THEREOF TO A TEMPERATURE BELOW THAT FOR STEAM FLASH AT THE REDUCED PRESSURE. 